At the time of performing a radiotherapy treatment, an object to be treated is positioned at the previously-planned position. There is a method of positioning the object to be treated. For example, corresponding points are designated on a first image captured by radiography at the time of planning a treatment and on a second image captured by radiography at the time of actual treatment, and the object to be treated is positioned based on a difference in position between the corresponding points.
It is difficult to designate the corresponding points when the difference is large in posture of the object between at the planning time and at the treatment time.